The synthesis of oligodeoxynucleotides containing a dihydro-5-azacytosine moiety instead of cytosine at any desired position of a sequence in an oligo can now be achieved routinely. A new and efficient preparative HPLC method for the purification of these oligodeoxynucleotides was also developed. Oxidation of the dihydro-5-azacytosine moiety to the aromatic 5-azacytosine has been attempted and continues to be investigated. The target oligodeoxynucleotides containing 5-azacytosine moieties are expected to behave as inhibitors of DNA methylase and to affect gene expression.